There is a great need for novel interventions of chronic osteomyelitis (OM) as approximately 112,000 orthopedic device-related infections occur per year in the US, at an approximate hospital cost of $15,000-70,000 per incident (Darouiche, “Treatment of Infections Associated With Surgical Implants,” N. Engl. J. Med. 350(14):1422-9 (2004)). Although improvements in surgical technique and aggressive antibiotic prophylaxis have decreased the infection rate following orthopedic implant surgery to 1-5%, osteomyelitis (OM) remains a serious problem and appears to be on the rise from minimally invasive surgery (Mahomed et al., “Rates and Outcomes of Primary and Revision Total Hip Replacement in the United States Medicare Population,” J. Bone Joint Surg. Am. 85(A-1):27-32 (2003); WHO Global Strategy for Containment of Antimicrobial Resistance, 2001). The significance of this resurgence, 80% of which is due to Staphylococcus aureus, is amplified by the fact that ˜50% of clinical isolates are methicillin resistant S. aureus (MRSA). While the infection rates for joint prosthesis and fracture-fixation devices have been only 0.3-11% and 5-15% of cases, respectively, over the last decade (Lew and Waldvogel, “Osteomyelitis,” Lancet 364(9431):369-79 (2004); Toms et al., “The Management of Peri-Prosthetic Infection in Total Joint Arthroplasty,” J. Bone Joint Surg. Br. 88(2):149-55 (2006)), this result may lead to amputation or death. Additionally, the popularization of “minimally invasive surgery” for elective total joint replacements (TJR) in which the very small incision often leads to complications from the prosthesis contacting skin during implantation, has markedly increased the incidence of OM (Mahomed et al., “Rates and Outcomes of Primary and Revision Total Hip Replacement in the United States Medicare Population,” J. Bone Joint Surg. Am. 85(A-1):27-32 (2003); WHO Global Strategy for Containment of Antimicrobial Resistance, 2001). These infections require a very expensive two-stage revision surgery, and recent reports suggest that success rates could be as low as 50% (Azzam et al., “Outcome of a Second Two-stage Reimplantation for Periprosthetic Knee Infection,” Clin. Orthop. Relat. Res. 467(7):1706-14 (2009)). However, the greatest concern is the emergence of drug resistant strains, most notably MRSA, which has surpassed HIV as the most deadly pathogen in North America, and continues to make the management of chronic OM more difficult, placing a great demand for novel therapeutic interventions. There is a great need for alternative interventional strategies, particularly for immune compromised elderly who are the primary recipients of TJR.
Presently, there are no prophylactic treatments that can protect high-risk patients from MRSA, most notably the aging “baby boomers” who account for most of the 1.5 million TJR performed annually in the United States. A vaccine that would decrease the MRSA incidence by 50-80% would not only reduce the number one complication of joint replacement and open fracture repair procedures, but also cut the healthcare burden by a similar amount.
Studies have documented that 80% of chronic OM is caused by S. aureus. These bacteria contain several factors that make them bone pathogens including several cell-surface adhesion molecules that facilitate their binding to bone matrix (Flock et al., “Cloning and Expression of the Gene for a Fibronectin-Binding Protein From Staphylococcus aureus,” Embo. J. 6(8):2351-7 (1987)), toxins capable of stimulating bone resorption (Nair et al., “Surface-Associated Proteins From Staphylococcus aureus Demonstrate Potent Bone Resorbing Activity,” J. Bone Miner. Res. 10(5):726-34 (1995)), through increased osteoclast activity (Marriott et al., “Osteoblasts Express the Inflammatory Cytokine Interleukin-6 in a Murine Model of Staphylococcus aureus Osteomyelitis and Infected Human Bone Tissue,” Am. J. Pathol. 164(4):1399-406 (2004)). The rate-limiting step in the evolution and persistence of infection is the formation of biofilm around implanted devices (Costerton et al., “Bacterial Biofilms: A Common Cause of Persistent Infections,” Science 284(5418):1318-22 (1999)). Shortly after implantation, a conditioning layer composed of host-derived adhesins (including fibrinogen, fibronectin, and collagen) forms on the surface of the implant and invites the adherence of free-floating bacteria derived from hematogenous seeding, including spread of infection from a contiguous area (the skin adjacent to a wound), surgical inoculation of bacteria into bone, or trauma coincident with significant disruption of the associated soft tissue bone envelope (Darouiche, “Treatment of Infections Associated With Surgical Implants,” N. Engl. J. Med. 350(14):1422-9 (2004)). Over the next few days bacterial cell division, recruitment of additional planktonic organisms, and secretion of bacterial products (such as the glycocalyx) produces the biofilm. This biofilm serves as a dominant barrier to protect the bacteria from the action of antibiotics, phagocytic cells, antibodies and impairs lymphocyte functions (Gray et al., “Effect of Extracellular Slime Substance From Staphylococcus epidermidis on the Human Cellular Immune Response,” Lancet 1(8373):365-7 (1984); Johnson et al., “Interference With Granulocyte Function By Staphylococcus epidermidis Slime,” Infect. Immun. 54(1):13-20 (1986); Naylor et al., “Antibiotic Resistance of Biomaterial-Adherent Coagulase-Negative and Coagulase-Positive Staphylococci,” Clin. Orthop. Relat. Res. 261:126-33 (1990)).
Another recent discovery is that S. aureus not only colonizes bone matrix, but is also internalized by osteoblasts in vitro (Ellington et al., “Involvement of Mitogen-Activated Protein Kinase Pathways in Staphylococcus aureus Invasion of Normal Osteoblasts,” Infect. Immun. 69(9):5235-42 (2001)) and in vivo (Reilly et al., “In Vivo Internalization of Staphylococcus aureus by Embryonic Chick Osteoblasts,” Bone 26(1):63-70 (2000)). This provides yet another layer of antibody and antibiotic resistance. This phase of infection occurs under conditions of markedly reduced metabolic activity and sometimes appears as so-called small-colony variants that likely accounts for its persistence (Proctor et al., “Persistent and Relapsing Infections Associated with Small-Colony Variants of Staphylococcus aureus,” Clin. Infect. Dis. 20(1):95-102 (1995)). At this point the bacteria may also express phenotypic resistance to antimicrobial treatment, also explaining the high failure rate of short courses of therapy (Chuard et al., “Resistance of Staphylococcus aureus Recovered From Infected Foreign Body in Vivo to Killing by Antimicrobials,” J. Infect. Dis. 163(6):1369-73 (1991)). Due to these extensive pathogenic mechanism, OM is notorious for its tendency to recur even after years of quiescence, and it is accepted that a complete cure is an unlikely outcome (Mader and Calhoun, “Long-Bone Osteomyelitis Diagnosis and Management,” Hosp. Pract. (Off Ed) 29(10):71-6,9,83 passim (1994)).
One of the key questions in the field of chronic OM is why current knowledge of factors that regulate chronic OM is so limited. Supposedly, the experimental tools necessary to elucidate bacterial virulence genes have been available for over a century. There are three explanations for this anomaly. First, although the total number of osteomyelitis cases is high, its incidence of 1-5% is too low for rigorous prospective clinical studies, with the possible exception of revision arthropasty. Second, it is well known that in vitro cultures rapidly select for growth of organisms that do not elaborate an extracellular capsule, such that biofilm biology can only be studied with in vivo models (Costerton et al., “Bacterial Biofilms: A Common Cause of Persistent Infections,” Science 284(5418):1318-22 (1999)). This leads to the “greatest obstacle” in this field, which is the absence of a quantitative animal model that can assess the initial planktonic growth phase of the bacteria prior to biofilm formation. To date, much of the knowledge of its pathogenesis comes from animal models (Norden, “Lessons Learned From Animal Models of Osteomyelitis,” Rev. Infect. Dis. 10(1):103-10 (1988)), which have been developed for the chicken (Daum et al., “A Model of Staphylococcus aureus Bacteremia, Septic Arthritis, and Osteomyelitis in Chickens,” J. Orthop. Res. 8(6):804-13 (1990)), rat (Rissing et al., “Model of Experimental Chronic Osteomyelitis in Rats,” Infect. Immun. 47(3):581-6 (1985)), guinea pig (Passl et al., “A Model of Experimental Post-Traumatic Osteomyelitis in Guinea Pigs,” J. Trauma 24(4):323-6 (1984)), rabbit (Worlock et al., “An Experimental Model of Post-Traumatic Osteomyelitis in Rabbits,” Br. J. Exp. Pathol. 69(2):235-44 (1988)), dog (Varshney et al., “Experimental Model of Staphylococcal Osteomyelitis in Dogs,” Indian J. Exp. Biol. 27(9):816-9 (1989)), sheep (Kaarsemaker et al., “New Model for Chronic Osteomyelitis With Staphylococcus aureus in Sheep,” Clin. Orthop. Belot. Res. 339:246-52 (1997)) and most recently mouse (Marriott et al., “Osteoblasts Express the Inflammatory Cytokine Interleukin-6 in a Murine Model of Staphylococcus aureus Osteomyelitis and Infected Human Bone Tissue,” Am. J. Pathol. 164(4):1399-406 (2004)). While these models have been used to confirm the importance of bacterial adhesions identified from in vitro assays (Chuard et al., “Susceptibility of Staphylococcus aureus Growing on Fibronectin-Coated Surfaces to Bactericidal Antibiotics,” Antimicrob. Agents Chemother. 37(4):625-32 (1993); Buxton et al., “Binding of a Staphylococcus aureus Bone Pathogen to Type I Collagen,” Microb. Pathog. 8(6):441-8 (1990); Switalski et al., “A Collagen Receptor on Staphylococcus aureus Strains Isolated From Patients With Septic Arthritis Mediates Adhesion to Cartilage,” Mol. Microbiol. 7(1):99-107 (1993)), they do not have an outcome measure of in vivo growth, bacterial load, or osteolysis. Thus, they cannot be efficiently used to assess drug effects, bacterial mutants, and the role of host factors with transgenic mice.
Based on over 150 years of research, a clear paradigm to explain microbial pathogenesis has emerged. This model also applies to OM. The initial step of infection occurs when a unicellular bacterium invades the body. At this point the microbe must respond to environmental changes and express virulence genes that will help it defeat innate immunity and provide it with adhesin receptors to attach to the host. The bacterium is also dependent on the stochastic availability of host adhesins from necrotic tissue or a foreign body such as an implant. Successful completion of these steps leads to an exponential growth phase, which ceases at the point of nutrient exhaustion and/or the development of adaptive immunity. Following the exponential growth phase the bacteria are forced to persist under dormant growth conditions within the biofilm. However, at this point the infection is now chronic and cannot be eradicated by drugs or host immunity. Thus, the focus in this field has been on cell surface adhesins that specifically interact with extracellular matrix components known as MSCRAMMs (microbial surface components recognizing adhesive matrix molecules) (Patti et al., “MSCRAMM-Mediated Adherence of Microorganisms to Host Tissues,” Annu. Rev. Microbiol. 48:585-617 (1994)). In fact, essentially all anti-S. aureus vaccines that have been developed to date have been directed against MSCRAMMs that are important for host tissue colonization and invasion. The goal of these vaccines is to generate antibodies that bind to these surface antigens, thereby inhibiting their attachment to host tissue. By opsinizing the bacterial surface, these antibodies can also mediate S. aureus clearance by phagocytic cells. Unfortunately, S. aureus has many adhesins, such that inhibition of one or more may not be sufficient to prevent bacterial attachment. Furthermore, bacterial clearance by phagocytic cells may be limited in avascular tissue, such that mAb may need additional anti-microbial mechanism of action to significantly reduce the in vivo planktonic growth of S. aureus and prevent the establishment of chronic OM or reinfection during revision total joint replacement surgery.
The present invention is directed to overcoming these and other deficiencies in the art.